The Design Concept of K-band Frequency Tripler

Martyna Skweres

Microwave Active Components and EMC Group

PIT-RADWAR S.A.

Warsaw, Poland

martyna.skweres@pitradwar.com

Abstract— In this paper the working principle and fundamental

properties of frequency multipliers are described. The article

presents the frequency tripler solution which generates microwave

signal response in the 18 GHz band (K-band). The various stages

of the designed tripler for the C-band input signal are introduced.

Test results including the third harmonic frequency response of

the manufactured tripler model are also presented.

Index Terms— frequency multiplier; frequency tripler; K-band

I. INTRODUCTION

There are several frequency multiplying techniques for generating harmonic frequencies from a fundamental input frequency. For example frequency doublers can be designed by exploiting the square-law current–voltage characteristic of field-effect transistors (FETs) or by using antiparallel diodes. The frequency multipliers such as triplers are more challenging to design and manufacture. Frequency multipliers, especially high-frequency triplers are offered only by few companies. These triplers are also custom-made what is associated with high costs.

II. THEORY

The frequency multipliers are used to obtain a certain high frequency signal. The output signal frequency is an integer multiple of the input signal frequency. The basic block diagram of a frequency multiplier is shown in Figure 1.

Figure 1. Diagram of a frequency multiplier

In order to obtain the output frequency that is an integer multiple of the input frequency, a component with non-linear characteristic should be used. The current dependence for the multiplier when the voltage is applied can be written by Taylor series as [2]:

i = Av + Bv2 + Cv3 + ⋯

where: i - current v - voltage

Assuming that the input voltage is a cosinusoidal signal it is

easy to predict that the higher order harmonics will be received

from the multiplier output. This is due to the following equation

[2]:

cos2x =1

2(1 + cos(2x))

cos3x =

1

2(cos(x) + cos(x) ∙ cos(2x)) =

1

2cos(x) +

1

4(cos(x) + cos(3x))

In order to isolate a number of desired harmonics from the output signal it is needed to use matched filters.

Frequency multiplier is a circuit that generates a sinusoidal signal with a frequency n-times (n=2, 3, 4,…) higher than frequency of the input signal. Multipliers can be divided according to the action principles [2]:

Synchronous multipliers base on the principle of the synchronization by the input signal with frequency f1 from the signal generator. The output resonant frequencies of the synchronous multiplier oscillations are close to fn = nf1, where n = 2, 3, 4,….

Resonant multipliers operate on the principle of the input signal distortion by the non-linear components (such as: transistors, diodes) and the selection of the appropriate input signal harmonic by the resonant circuit.

Resonant multipliers depended on the non-linear component can be divided into:

transistor duplicators,

duplicators with non-linear resistive two-port networks,

varactor duplicators.

The basic parameters of frequency multipliers are:

n-duplication factor:

n=fout

fin

978-1-5090-2214-4/16/$31.00 ©2016 IEEE

where: fin - frequency of the input signal,

fout - frequency of the obtained output signal.

multiplier efficiency is the ratio of the power level of the output signal P(fn) to the input signal power level P(f)

η=P(fn)

P(f) (5)

The output power level for the n-th harmonic is specified by the following equation [4]:

Pn <P1

n2 (6)

Along with a multiplication order increases the output power decreases, so high order multiplication should be avoided. Additionally the relative frequency difference between the components ((n − 1)f1, nf1, (n + 1)f1) decreases at high multiplication, what is causing the demand for a high resonance quality factor of circuits and limits the possible tuning range of multipliers. The output resonant circuit is tuned to a multiple of the input frequency 𝑛𝑓1 and it is stimulated to oscillate once to n-periods of the output signal.

III. FREQUENCY TRIPLER DESIGN

The structure of the designed frequency tripler is shown below. It was assumed that the input frequency is 6 GHz (C-band), thus output frequency is equal to 18 GHz (K-band).

In order to meet requirements mentioned above, the appropriate frequency tripler structure was designed. The diagram of this structure is shown in Figure 2.

Figure 2. General structure of the frequency tripler

The frequency tripler circuit includes non-linear element that is antiparallel diode pair. In fact such element can be considered as a microwave two-port network, which is connected in series between the input and output matching circuits and the output filter. The M/A-COM diodes MA4E2508 were used in this structure. They can operate up to maximum frequency of 26 GHz. Casing and arrangement of diode model pads are shown in Figure 3, while in Table 1 the component dimensions are presented (according to the producer technical documentation). Both input and output paths of the frequency tripler are matched to 50 ohms. The output matching and filtering circuits of the designed multiplier were made mainly in order to obtain a reduced power level of undesirable harmonics at the multiplier output.

Figure 3. Casing of nonlinear component

TABLE I CASING DIMENSIONS

Dimension Inches Millimeters

Min. Max. Min. Max.

A 0,0445 0,0465 1,130 1,180

B 0,0169 0,0189 0,430 0,480

C 0,0040 0,0080 0,102 0,203

D 0,0128 0,0148 0,325 0,375

E 0,0128 0,0148 0,325 0,375

The schematic diagram of the designed frequency tripler is shown in Figure 4. Two filters at the output of multiplier (after the non-linear component) are band-pass filters with a center frequency of 18 GHz. The bandwidth of these filters is 1 GHz. Both filters are made in microstrip technology and their intended to transfer the third harmonic of the input signal (18 GHz). Additionally, the proposed structure includes attenuators (RCAT 03+) and active components such as amplifiers which perform the function of matching circuits, and provide the required power level of the third harmonic at the multiplier output. The use of the proposed elements enable to increase the efficiency of the tripling process. In the arrangement of the designed tripler, monolithic amplifiers are used to amplify the signal given at their inputs. In the designed circuit the amplifier AVA-183A+ from Mini-Circuits was applied. These unit operates in the frequency range from 5 GHz up to 18 GHz.

Figure 4. Schematic diagram of frequency tripler

The proposed tripler structure was manufactured on the laminate TLX-9-0250-CH/CH Taconic, which is appropriate for high-frequency applications. The laminate TLX-9 Taconic was used in the design and execution of microstrip filters due to the low loss factor δ = 0,0022 and the dielectric constant of the substrate Ԑ𝑟 = 2,5. The relative dielectric Ԑ𝑟 of the substrate

influences on the copper path width, where with the increase of Ԑ𝑟, the filter size decreases.

Band-pass filters are commonly used in microwave systems in order to eliminate the adverse interference and intermodulation products. Due to the limited performance of capacitors and inductors, the microwave band-pass filters are most often constructed using strip resonators o volumetric resonators. The main strip resonators are quarter-wave or half-wave sections of the homogeneous guides, which shall be shorted or opened at the end. The output band-pass filter was designed in the asymmetrical stripline technology. The initial design requirements for the filter were given as follows:

Bandwidth: 17 500 - 18 500 MHz;

The band-stop attenuation: 30 - 40 dB;

Return loss: less than - 20 dB;

Chebyshev characteristics with waviness of 0.2 dB;

Input and output impedance matched to 50 Ω filter.

The simulation results for the band-pass filter are shown in Figure 5.

Figure 5. Simulation results for the band-pass filter

Figure 6 shows the microstrip filter structure manufactured on a digital milling machine "Quick Circuit".

Figure 6. Band-pass filter 18 GHz in microstrip technology

The frequency tripler structure is divided into separate

functional units. This action allows to evaluate the impact of

each individual unit on the key performance of the designed

tripler. The frequency tripler circuit was tested. For this purpose

all individual functional units were connected through the SMA

connectors. The final model layout is shown in Figure 7.

Figure 7. Model of the frequency tripler

Test results for the input signal of 6 GHz frequency and 5 dBm input power are shown in Figure 8. The output power level after the filter for the third harmonic is equal to -19,58 dBm. The undesirable reproduced harmonics are filtered and their power level is reduced to -74 dBm.

Figure 8. Spectral-response characteristic at the output of frequency tripler for

fin= 6 GHz and Pin= 5 dBm

Figure 9 shows the characteristics of the output power level in function of the input power level for three harmonics produced by the frequency tripler circuit. This diagram illustrates the operation of the multiplier. Filters suppress the undesired harmonics properly to a level below -75 dBm. The third harmonic is transmitted with no significant attenuation. The output power level increases with the increase of the input power level and reaches a maximum (-16,10 dBm) for the input power Pin = 10 dBm.

Figure 9. Output power characteristics as a function of the input power of the

frequency tripler for the three harmonics (I harmonic, II harmonic, III

harmonic)

Date: 3.JUN.2015 09:27:38

The circuit was made on a common substrate. For this purpose,

the printed circuit board was designed in the Altium Designer.

Figure 10 presents the view of the PCB with the placement of

components on the board in the configuration of Altium's 3D.

Figure 10. Location of components on a PCB (3D view)

Designations in Figure 10 are in accordance with the schematic diagram shown in Figure 4.

IV. CONCLUSION

The main purpose of this article was to present the structure and the basic principles of the frequency multiplier. The laboratory model of such frequency tripler generating 18 GHz output signal was designed and manufactured. The presented circuit can be used as a signal microwave source in the transceiver devices dedicated for radar applications operating in the upper frequency ranges of Ku-band or lower ranges of K-band. The final model of the presented tripler was made on the common substrate, which allowed to eliminate SMA connectors and gave the ability to integrate the structure in a single housing.

REFERENCES

[1] S. Maas, Nonlinear Microwave circuits, Artech House, 1998.

[2] J. Boksa, Analogowe układy elektroniczne, wyd. BTC Warszawa 2007.

[3] A. W. J. Chramiec, Mikrofalowe mieszacze diodowe, WKŁ, Warszawa 1975.

[4] M. T. Faber, J. Chramiec, M. E. Adamski, Microwave and Millimeter- Wave Diode Frequency Multipliers, Artech House, Norwood 1995.